December 1S95.] 



PS re HE. 



313 



a rec(>giiizal)le consistency. Broadly 

 stated, the manner of the branchhig is 

 this : the stem forks rather near its base, 

 the upper branch, which eitliei' does 

 not fork again (in more specialized 

 wings) or gives oft" a few branches (a 

 mure generalized condition) appearing 

 to be more ilirectly a continuation of 

 the basal trunk than the lower branch, 

 which usuallv displays a "branching 

 away " character, and which is almost 

 always repeatedly foi'ked and branched. 

 This repeatedl}' foiking lower liranch 

 is the radial sector of authors. In the 

 more generalized venation the sector 

 branches tVom the radial stem near its 

 base, and is man\-forked. The modi- 

 lications which the sector antl its 

 branches exhibit, due to the specializing 

 tendency of the wingtoward narrowness 

 with accompanying coalescence and 

 disappearance of vein branches are the 

 reduction in the number (coalescence) 

 of the branches and the movement of 

 the point of origin of the sector farther 

 and farther away from the base of the 

 wing. 



Xow although the Ephemerid wings 

 are in point of specialization in advance 

 (shown by the reduction of the hind 

 wing, and the specialization of the 

 thorax) of the general rank of the 

 familv among in.sects (paired genital 

 openings, etc.), the wings have by no 

 means reached that degree of special- 

 ization where radius has become an 

 unbranched vein. In fact, radius in 

 the Ephemerid wing is, to my mind, in 

 very generalized condition. The many- 

 branched radial sector departs from the 

 stem verv near its base, so near indeed. 



that by a slight modification it has 

 become apparently entirely distinct from 

 radius, and, in some Ma\'-flies even 

 apparently joined at its base with media. 

 Such an apparent or even real dissocia- 

 tion of a branch from its original stem 

 and re-association with another vein is 

 not an uncommon phenomenon in the 

 modification of venation ; note among 

 the Lepitloptera the association of the 

 branches of media, after the base of 

 media has disappeared, with radius and 

 cubitus. 



Nor is this condition of radius and 

 its sector imique with the May-flies. 

 Among Neuropteroid insects in general 

 the sector usually arises near the base 

 of the radial stem (Odonata, Sialidae 

 ct a/.), and not untlequently is appar- 

 ently dissociated from the radial stem, 

 and re-as.sociated with media, as in 

 certain Odonata and Perlidae, and, 

 among unrelated forms, in Embia, 

 Fulgora c/ al. In some cases the base 

 of media is intimatel}' united (coalesces) 

 with the base of radius, as in Nemura 

 (see figure 3). In fact the crowding 

 together of the vein stems at the base of 

 the wing brings about much distortion 

 and modilication of these one-time 

 mutually independent anti co-important 

 trunks. 



